In the operation of furnaces, particularly blast furnaces but also direct reduction shaft or cupola furnaces, crucible furnaces, for example for coal gasification, ladle furnaces or low shaft furnaces for melting ferrous alloys, non-ferrous metals or non-metals, the taphole must be opened from time to time so that the melt or the by-products, e.g. slag, can run off. Depending on the process, the tapping cycle can vary between half an hour and several days. After tapping, the tapholes are closed again with plugging material.
It is known from EP-OS 41 942, when alternatively closing and opening the tapholes of blast furnaces, to drive a rod with a pneumatic hammer through the still not fully hardened plug until its tip reaches the interior of the blast furnace, i.e. the molten charge. While the front end of the rod that is in contact with the molten metal melts away, the rest of the rod remains in the bore hole and is only removed by pulling it out when the furnace is tapped. This so-called "back-pressure drilling process" has been found to be particularly advantageous when the furnace has a thick refractory lining any there are tapholes 2.5 m or more in length.
The success of the back-pressure drilling process depends essentially on precisely determining in advance the right time for driving in the tapping rod. Particular attention must be paid to forcing the plastic plugging material into the taphole under high pressure when tapping is complete, so that the molten material still in the taphole is forced back and the taphole passage is completely filled with the plugging material. This is done by means of a notch gun or taphole plugging machine, which must remain in front of the closed taphole opening for some time after the plugging so as to apply the necessary counterpressure until the plugging material begins to harden. This time depends on the composition of the plugging material, and also to an important extend on the temperature in the taphole and of the refractory lining. The gun is only swung from its operating position to its inoperative position when the material has hardened sufficiently to withstand the pressure in the interior of the furnace, i.e. for it not to be forced out again. In contrast to this requirement, or necessity, is the fact that the tapping rod can only be driven into the taphole while the plugging material in the taphole still has a certain amount of plasticity.
If however the tapping rod is driven into the plastic material too soon, it may happen that an unplanned escape of molten material occurs, with great danger to the operators. To avoid the risk of an unplanned breakthrough of molten material, the operators take care to wait rather longer than necessary before driving in the tapping rod. But too long a delay often results in the driving force of the pneumatic hammer being insufficient to drive the rod into the plugging material, which is then too hard. The back-pressure technique can then no longer be used, and the furnace must be drilled out again using the conventional single- or two-stage drilling process.
Because of the differences in composition of the commercially available plugging materials their hardening time varies a great deal, and in addition it changes with the duration and temperature of storage of the material before use. In many cases the period within which the driving rod ought to, or can, be driven in amounts only to one or two minutes, so that the number of failures is correspondingly large, with the result that the taphole frequently has to be drilled out. In the two-stage drilling process this is done by pre-drilling with a drill bit and then driving a smooth rod through. The relatively expensive drill bit can then be re-used. In single-stage drilling, on the other hand, the refractory lining is drilled through using a cheap bit that can only be used once. Both drilling processes have the disadvantage that the time required for drilling is quite long. In addition annealing of the drill bit may occur, which is particularly disadvantageous in the case of the expensive drill bit used in the two-stage process. Furthermore there is a risk that the drill rod may remain stuck in the taphole and--since it cannot be pulled out again--have to be burned out with an oxygen lance.